Light emitting device driving circuit

ABSTRACT

The circuit comprises an amplitude setting transistor  5 Q for controlling the amplitude of high-frequency current I 2 QB flowing through the second current mirror circuit  2 , by using an input of reference direct current signal Bias. Direct current component I 4 QB generated based on reference direct current signal Bias is subtracted from direct current I 1  flowing through the other side line of the first current mirror circuit  1 . In this case, level fluctuation of driving current IZ can be significantly suppressed, since the increment of the direct current component included in high-frequency current I 2 QB is proportional to direct current component I 4 QB which is subtracted from direct current I 1  flowing through said other side line of the first current mirror circuit  1.

TECHNICAL FIELD

[0001] The present invention relates to light emitting device drivingcircuits.

BACKGROUND ART

[0002] An Optical pickup is a device which can read information storedon storage media such as CDs or a DVDs by irradiating laser light fromlaser diodes onto the storage media and monitoring the light reflectedtherefrom. Laser diodes may also be used for writing information onstorage media. Heretofore, driving circuits for such laser diodes havebeen developed.

DISCLOSURE OF THE INVENTION

[0003] In an optical pickup, unnecessary light or background lightreflected in the apparatus may enter the photo-detector to become noisecontained in required information. In addition, when the light reflectedin storage media returns to a light emitting device and enters the lightemitting device, light emission of the light emitting device may becomeunstable, causing a noise.

[0004] Accordingly, it is conceivable that an approach for driving lightemitting device at high-frequency causes noise tolerance to improve.High-frequency current is superimposed onto direct current componentsand supplied to light emitting device as driving current. The preferredfrequency of high-frequency current applicable to present CD players isof 300-500 MHz.

[0005] It is also conceivable not to provide such high-frequency currentto the light emitting device to control power consumption. In addition,the mean value of high-frequency current in itself may also varydepending on temperature and aged deterioration. Thus, as the mean valueof the high-frequency current varies, the level of the driving currentchanges. Since a slight level fluctuation may increase the probabilityof detection errors, smaller level fluctuation is preferred.

[0006] The present invention is achieved in consideration of theabove-mentioned problems, and therefore an object of the presentinvention is to provide a light emitting device driving circuit capableof suppressing level fluctuation of the driving current in a state,where the light emitting device driving circuit superimposeshigh-frequency current components onto direct current components togenerate a driving current which in turn is supplied to the lightemitting device.

[0007] In order to address the above-mentioned problems, a lightemitting device driving circuit according to the present inventioncomprising a first and a second current mirror circuits each of whichhaving a pair of parallel lines in a state, where each one side line ofsaid parallel lines is connected to said light emitting device; applyingdirect current and high-frequency current respectively to the other sidelines of the line pairs in the first and the second current mirrorcircuits; and supplying, through a node of the connection, drivingcurrent generated by superimposing high-frequency current onto directcurrent, wherein, there is provided an amplitude setting transistor forcontrolling, by using an input of reference direct current signal, theamplitude of the high-frequency current flowing through the secondcurrent mirror circuit so that direct current components generated basedon the reference direct current signal may be subtracted from the directcurrent flowing through said other side line of said line pair of thefirst current mirror circuit.

[0008] More specifically, as the amplitude of the high-frequency currentin the second current mirror circuit increases, the direct currentcomponent of the high-frequency current increases, thereby increasingthe mean value of the driving current given as the sum of the currentsrespectively flowing through each one side line of the first and thesecond current mirror circuits.

[0009] When reference direct current signal which is input into theamplitude setting transistor increases, the amplitude of thehigh-frequency current flowing through the second current mirror circuitincreases thereby increasing the mean value of the driving current,while fluctuation of the mean value of the driving current may becontrolled because the direct current component generated from thereference direct current signal is subtracted from the direct currentflowing through the other side line of the first current mirror circuit.That is to say, since the driving current is generated by superimposingthe direct current which is equivalent or proportional to the directcurrent flowing through the other side line of the first current mirrorcircuit onto the high-frequency current, the above-mentioned directcurrent component is subtracted from the driving current, wherebyfluctuation of the mean value of the driving current can be suppressed.

[0010] In addition, level fluctuation of the driving current can besignificantly suppressed when the increment of the direct currentcomponent of said high-frequency current is set to be proportional tothe direct current component which will be subtracted from the directcurrent flowing through said other side line of said first currentmirror circuit.

[0011] A light emitting device driving circuit of the present inventioncomprises a variable current source for providing current to a node onsaid side line in said first current mirror circuit, and preferablycomprises a constant current source for keeping the amount of currentsupplied from this side line and said variable current source at aconstant value.

[0012] In this case, depending on the amount of the current suppliedfrom the variable current source, the current flowing through one andthe other side lines of the first current mirror circuit, i.e., the sizeof the direct current component supplied to the light emitting device,can be controlled.

[0013] Furthermore, in the light emitting device driving circuit of thepresent invention, said variable current source is composed of a currentmirror circuit, with a transistor disposed downstream of the input sideline of the current mirror circuit, wherein upon application of acurrent control voltage to the control terminal of the transistor, theamount of the current supplied from the variable current source, i.e.,the size of the direct current component supplied to the light emittingdevice can be controlled, depending on the level of the current controlvoltage.

[0014] In addition, the light emitting device driving circuit of thepresent invention comprises a high-frequency generation circuit forgenerating said high-frequency current, said amplitude settingtransistor preferably being connected to said high-frequency generationcircuit so that the upstream electric potential thereof determines theamplitude of said high-frequency current.

[0015] In this case, the amplitude of the high-frequency currentcomponent supplied to the light emitting device can be controlled byadjusting the upstream electric potential of the amplitude settingtransistor.

[0016] In the light emitting device driving circuit of the presentinvention, a common-mode voltage is preferably input into the controlterminal of said transistor disposed downstream of the input side lineof said variable current source, and the control terminal of saidamplitude setting transistor.

[0017] In this case, direct current and high-frequency current suppliedto the light emitting device can be altered in phase, since the inputvoltage to the control terminal of said transistor and the input voltageto the control terminal of said amplitude setting transistor have acommon mode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a circuit diagram of the light emitting device drivingcircuit.

[0019]FIG. 2 is a graph of current I2QA flowing in one of the side linesof the high-frequency generation circuit 3.

[0020]FIG. 3 is a graph illustrating the wave form of driving currentIZ.

BEST MODES FOR CARRYING OUT THE INVENTION

[0021] The light emitting device driving circuit according to anembodiment is described below. Like elements will be designated by likenumerals and redundant description will be omitted.

[0022]FIG. 1 is a circuit diagram of a light emitting device drivingcircuit. The current mirror circuit, composed by connecting the controlinput terminals of two transistors, has two parallel lines through whichcurrents of the transistors flow respectively.

[0023] A plurality of current mirror circuits 1, 2 and 4 are describedin the following description, wherein current mirror circuits 1, 2 and 4respectively have, as shown in the case of FET, transistor pairs 1QA,1QB; 2QA, 2QB; 4QA, 4QB, each gate thereof being connected as thecontrol input terminal. Here, in the case of a bipolar transistor, thecontrol input terminal functions as the base.

[0024] The present driving circuit comprises a first current mirrorcircuit 1 (circuit for supplying direct current) and a second currentmirror circuit 2 (circuit for supplying high-frequency current), eachhaving a pair of parallel lines. Each one side line in current mirrorcircuits 1 and 2 is connected to a light emitting device(load) Z.

[0025] By passing direct current and high-frequency current respectivelythrough the other side lines of said parallel lines in the first and thesecond current mirror circuits 1 and 2, the present driving circuitsupplies driving current IZ generated by superimposing direct currentonto high-frequency current to light emitting device Z, through aconnection node Y.

[0026] First the configuration of the part of direct current will bedescribed. A direct current source I1 (from which current I1 may flow)is connected at a downstream region of the other side line in the firstcurrent mirror circuit 1, so that direct current component I4QB flowsbetween this side line and the current source I1 through node X.Accordingly, current I1QA, generated as a result of subtracting directcurrent component I4QB from direct current I1, flows through an upstreamregion beyond node X of the side line, whereas current I1QB which isequivalent or proportional to current I1QA flows through the counterpartside line.

[0027] In essence, direct current I1QB supplied to light emitting deviceZ is generated as a result of subtracting a predetermined direct currentI4QB from direct current I1. The determined direct current componentI4QB is generated in current mirror circuit 4 for generating a directcurrent component. In other words, direct current component I4QA whichis equivalent or proportional to direct current component I4QB flowsthrough transistor 4QA which is the counterpart of transistor 4QBthrough which direct current component I4QB flows. Direct currentcomponent I4QA is proportional to a reference direct current signal(control voltage) Bias which is input to the control input terminal(gate) of direct current setting transistor 6Q disposed downstream oftransistor 4QA.

[0028] Accordingly, direct current component I4QA (I4QB) generated basedon reference current signal Bias is subtracted from direct current I1flowing through the other of said side lines in the first current mirrorcircuit 1.

[0029] The configuration of the high-frequency side will be describednext. Reference direct current signal Bias is also input into thecontrol input terminal (gate) of amplitude setting transistor 5Q,thereby passing direct current I5Q through amplitude setting transistor5Q. Direct current I5Q flowing through amplitude setting transistor 5Qis proportional to the amplitude of high-frequency current generated inhigh-frequency generation circuit (differential current switch) 3.

[0030] In other words, high-frequency generation circuit 3 has anoscillating circuit OSC for mediation between the control inputterminals (gates) of a pair of transistors 3QA and 3QB, with anamplitude setting transistor 5Q being connected to the downstream sideof transistors 3QA and 3QB to determine the aggregate sum (amplitude,direct current component) of currents I3QA and I2QA flowing throughtransistors 3QA and 3QB, respectively.

[0031]FIG. 2 is a graph showing current I2QA flowing through one of theside lines of high-frequency generation circuit 3. High-frequencygeneration circuit 3 completely separates current I5Q into I3QA and I2QAaccording to the phase of an oscillating circuit OSC. Thus, I2QA becomesa pulse current having a peak value corresponding to I5Q, andconsequently, has a direct current component of I5Q/2.

[0032] Current I2QA flowing through transistor 3QB, one of the twotransistors in the high-frequency generation circuit 3, is equivalent tothe current flowing through said other side line of the second currentmirror circuit 2 for high-frequency. Thus, current I2QB which isequivalent or proportional to current I2QA flows through transistor 2QBand is supplied to light emitting device Z through node Y, transistor2QB being the counterpart of transistor 2QA through which current I2QAflows.

[0033] In essence, amplitude setting transistor 5Q controls theamplitude of high-frequency current I2QA(I2QB) flowing through thesecond current mirror circuit 2 according to the input of referencedirect current signal Bias.

[0034] When the amplitude of high-frequency current I2QA(I2QB) in thesecond current mirror circuit 2 increases, the direct current componentof which increases, and consequently the mean value of driving currentIZ given as the sum of the currents respectively flowing through eachone side line of both the first and the second mirror circuits 1, 2increases.

[0035] When reference direct current signal Bias which is input intoamplitude setting transistor 5Q increases, the amplitude ofhigh-frequency current I2QA(I2QB) flowing through the second currentmirror circuit 2 increases and the mean value of driving current IZincreases, while fluctuation of the mean value of driving current IZwill be suppressed since direct current component I4QA(I4QB) generatedfrom reference direct current signal Bias is subtracted from directcurrent I1 flowing through the other side line of the first currentmirror circuit 1.

[0036] In other words, since driving current IZ results fromsuperimposing direct current I1QB which is equivalent or proportional todirect current I1QA flowing through the other side line of the firstcurrent mirror circuit 1 onto high-frequency current I2QB, directcurrent component I4QA(I4QB) is subtracted from driving current IZ,whereby fluctuation of the mean value of driving current can besuppressed.

[0037] Here, description will be provided about the current flowingthrough each transistor. Let us assume that I1QB=G1*I1QA, I2QB=G2*I2QA,I4QB=G4*I4QA, where G1, G2, and G4 are gain ratios of the transistors,with a value of 1, for example. I1QB=I1−I4QB holds, since I1QB and I4QBwill be added at node X to give I1. Here, driving current IZ is anaddition of currents I1QB and I2QB, each current being added at node Y.

[0038] Transistors 5Q and 6Q have a common gate, which is connected toreference direct current signal Bias. Each current is controlled byreference direct current signal Bias. Let coefficient G56 be the gainratio between transistors 5Q and 6Q, the size of each transistor isdetermined so that the current ratio will be I6Q=G56*I5Q. The amplitudeof high-frequency current is controlled by reference direct currentsignal Bias.

[0039] In other words, $\begin{matrix}{{IZ} = {{I1QB} + {I2QB}}} \\{= {{{G1}*{I1QA}} + {I2QB}}} \\{= {{{G1}*\left( {{I1} - {I4QB}} \right)} + {I2QB}}} \\{= {{{G1}*\left( {{I1} - {{G4}*{I4QA}}} \right)} + {{G2}*{I2QA}}}} \\{= {{{G1}*\left( {{I1} - {{G4}*{G56}*{I5Q}}} \right)} + {{G2}*{I2QA}}}} \\{= {{{G1}*{I1}} - {{G1}*{G4}*{G56}*{I5Q}} + {{G2}*{I2QA}\quad {{holds}.}}}}\end{matrix}$

[0040] Let <IZ> denote the temporal mean value of driving current IZ,then,

<IZ>=G 1*I 1−G 1*G 4*G 56*I 5 Q+G 2*(I 5 Q/2)

[0041] holds (equation A). Here, the temporal mean value of IZ (level)is set so as not to be varied by reference direct current signal Bias.In other words, it is sufficient that the second and the third terms ofequation A cancel each other in order to keep the temporal mean value ofdriving current IZ invariant against the change of the amplitude ofhigh-frequency current.

[0042] Thus, G1*G4*G56=G2/2 holds. In this case <IZ>=G 1*I1. G2=2 holdsassuming that G1=G4=G56=1.

[0043] In other words, level fluctuation of driving current IZ can besignificantly suppressed, since the increment (caused by the increase ofamplitude) of the direct current component included in high-frequencycurrent I2QB is set to be proportional (or equivalent) to direct currentcomponent I4QB which is subtracted from direct current I1 flowingthrough the other side line of the first current mirror circuit 1.

[0044]FIG. 3 is a graph showing the waveform of driving current IZ whenthe above equations hold. As can be seen from the graph, mean current<IZ> is invariant against the change of the amplitude of driving currentIZ.

[0045] As described above, since fluctuation of the temporal mean valueof driving current IZ is suppressed in the above mentioned lightemitting device driving circuit, the intensity of laser light does notvary regardless of the presence of high-frequency superimposing when thelight emitting device driving circuit is driven with light emittingdevice Z as the laser diode, which is particularly effective when usinghologram in the optical system.

[0046] In addition, for an optical disk memory, laser light isirradiated onto the disk surface to read information. Laser diodes areused as the laser source, in which case reflected light from the diskreturns to the LD, causing noise. The above-mentioned light emittingdevice driving circuit is also effective when superimposinghigh-frequency (300-500 MHz) as a solution since level fluctuation issuppressed.

[0047] As described above, the above-mentioned light emitting devicedriving circuit comprises a variable current source 4 for supplyingcurrent to node X on the other side line in the first current mirror iscircuit 1, and a constant current source I1 for preserving the sum ofcurrent value I1QA flowing through this side line and current value I4QBsupplied from variable current source 4 to a constant value.

[0048] In this case, depending on the amount of current I4QB suppliedfrom variable current source 4, the amount of current flowing throughone and the other side line of the first current mirror circuit 1, i.e.,the size of direct current component supplied to light emitting device Zcan be controlled.

[0049] In addition, variable current source 4 is composed of currentmirror circuit 4, with light emitting device driving circuit having atransistor 6Q disposed downstream of input side line of current mirrorcircuit 4, and when current control voltage Bias is applied to thecontrol terminal (gate) of transistor 6Q, the amount of current I4QBsupplied from variable current source 4, i.e., the size of directcurrent component supplied to light emitting device Z can be controlleddepending on the size of this current control voltage Bias.

[0050] In addition, the light emitting device driving circuit of thepresent invention comprises high-frequency generation circuit 3 forgenerating high-frequency current, wherein amplitude setting transistor5Q is connected to high-frequency generation circuit 3 so that theupstream electric potential determines the amplitude of high-frequencycurrent.

[0051] Thus, the amplitude of high-frequency current component suppliedto light emitting device Z can be controlled by adjusting upstreamelectric potential of transistor 5Q using voltage Bias.

[0052] In addition, in the light emitting device driving circuit, acommon mode voltage Bias is input into the control terminal oftransistor 6Q which is disposed downstream of the input side line ofvariable current source 4 and the control terminal of amplitude settingtransistor 5Q.

[0053] In this case, direct current and high-frequency current suppliedto light emitting device Z can be altered in phase, since input voltageBias to the control terminal of transistor 6Q and input voltage Bias tothe control terminal of amplitude setting transistor 5Q have a commonmode.

[0054] As described above, level fluctuation of driving current can besuppressed according to the light emitting device driving circuit of thepresent invention.

INDUSTRIAL APPLICABILITY

[0055] The present invention can be used for light emitting devicedriving circuits.

1. A light emitting device driving circuit comprising: a first and asecond current mirror circuits each of which having a pair of parallellines in a state, where each one side line of said parallel lines isconnected to said light emitting device; applying direct current andhigh-frequency current respectively to the other side lines of the linepairs in said first and second current mirror circuits; and supplying,through a node of the connection, driving current generated bysuperimposing high-frequency current onto direct current, wherein, thereis provided an amplitude setting transistor for controlling, by using aninput of reference direct current signal, the amplitude of thehigh-frequency current flowing through said second current mirrorcircuit so that direct current components generated based on saidreference direct current signal may be subtracted from the directcurrent flowing through said other side line of said line pair of saidfirst current mirror circuit.
 2. A light emitting device driving circuitaccording to claim 1, wherein the increment of the direct currentcomponent of said high-frequency current is set to be proportional tothe direct current component subtracted from the direct current flowingthrough said other side line of said first current mirror circuit.
 3. Alight emitting device driving circuit according to claim 1, comprising avariable current source for supplying current to a node in said otherside line in said first current mirror circuit, and a constant currentsource for preserving the current value supplied from this line and saidvariable current source to a constant value.
 4. A light emitting devicedriving circuit according to claim 3, said variable current source beingcomposed of a current mirror circuit, with a transistor disposeddownstream of input side line of the current mirror circuit, wherein acurrent control voltage is applied to the control terminal of thetransistor.
 5. A light emitting device driving circuit according toclaim 4, comprising a high-frequency generation circuit for generatingsaid high-frequency current, wherein said amplitude setting transistoris connected to said high-frequency generation circuit so that theupstream electric potential determines the amplitude of saidhigh-frequency current.
 6. A light emitting device driving circuitaccording to claim 5, wherein a common mode voltage is input to thecontrol terminal of said transistor disposed downstream of the inputside line of said variable current source, and the control terminal ofsaid amplitude setting transistor.